Retroreflective articles have the ability to redirect incident light back towards the light source. This unique ability has led to the wide-spread use of retroreflective articles on various substrates. For example, retroreflective articles can be used on flat inflexible substrates, such as road signs and barricades; on irregular surfaces, such as corrugated metal truck trailers, license plates, and traffic barriers; and on flexible substrates, such as road worker safety vests, a jogger""s shoes, roll up signs, and canvas-sided trucks.
There are two major types of retroreflective articles: beaded articles and cube-corner articles. Beaded articles commonly use a multitude of glass or ceramic microspheres to retroreflect incident light. Typically, the microspheres are partially embedded in a support film, and a specular reflecting material is provided between the layer of microspheres and the support film. The reflecting material can be a metal layer (for example, an aluminum coating as disclosed in U.S. Pat. Nos. 3,700,478 and 4,648,932) or an inorganic dielectric mirror made up of multiple layers of inorganic materials that have different refractive indices (as disclosed in U.S. Pat. Nos. 3,700,305 and 4,763,985). Categories of beaded articles include exposed lens, enclosed lens, and encapsulated lens types. Exposed lens beaded articles have a layer of microspheres that are exposed to the environment. Enclosed lens beaded articles have a protective layer such as a transparent polymer resin contacting and surrounding the front side of the microspheres. Encapsulated lens articles have an air gap surrounding the front side of the microspheres and have a transparent film hermetically sealed to a support film to protect the microspheres from water, dirt, or other environmental elements.
In lieu of microspheres, cube-corner sheeting typically employs a multitude of cube-corner elements to retroreflect incident light. The cube-corner elements project from the back surface of a body layer. In this configuration, incident light enters the sheet at a front surface, passes through the body layer to be internally reflected by the faces of the cube-corner elements, and subsequently exits the front surface to be returned towards the light source. Reflection at the cube-corner faces can occur by total internal reflection when the cube-corner elements are encased in a lower refractive index media (e.g., air) or by reflection off a specular reflective coating such as a vapor deposited aluminum film. Illustrative examples of cube-corner sheeting are disclosed in U.S. Pat. Nos. 3,712,706; 4,025,159; 4,202,600; 4,243,618; 4,349,598; 4,576,850; 4,588,258; 4,775,219; and 4,895,428.
The present invention provides a new approach to supplying retroreflective articles with reflective coatings. In brief summary, the present invention provides a retroreflective article that comprises: (a) a layer of optical elements; and (b) a reflective coating that is disposed in optical association with the optical elements, the reflective coating comprising a plurality of layers wherein (i) at least two adjacent layers have different refractive indices, and (ii) the reflective coating includes multiple polymer layers that each have an average thickness that is less than about 10% of an average size of the optical elements.
Retroreflective articles of this invention differ from known retroreflective articles in that the optical elements have an associated reflective coating that comprises multiple polymer layers. The polymer layers can have indices of refraction and thicknesses selected such that the overall multilayer reflective coating reflects light in a desired wavelength range. Known retroreflective articles have used metal reflective layers, which in some instances can be subject to oxidation from air or moisture. When oxidized, the reflective layer can suffer a substantial loss in its reflective ability. Retroreflective articles have also employed multilayered inorganic dielectric mirrors that can be susceptible to air or moisture induced corrosion that can degrade reflectivity and/or lead to delamination of the layers. The polymer multilayer reflective coating of the present invention is beneficial in that it can be made highly reflective to light in a desired wavelength band(s), while also being capable of resisting undesirable environmental effects, such as air and/or moisture induced corrosion, to which known inorganic reflective coatings can be susceptible. The multilayer reflective coating of the present invention can also include inorganic and/or non-polymer layers disposed adjacent to or between the multiple polymer layers, for example, to help overcome limitations of known inorganic reflector coatings by rendering them more resistant to water, acids, bases, corrosion or other environmental degradation.
The above and other advantages of the invention are more fully shown and described in the drawings and detailed description of this invention. It is to be understood, however, that the description and drawings are for illustrative purposes and should not be read in a manner that would unduly limit the scope of the invention.
As used in this document, the following terms have the following definitions:
xe2x80x9cIndex of refractionxe2x80x9d or xe2x80x9crefractive indexxe2x80x9d is a material property that represents the ratio of the phase velocity of an electromagnetic wave in a vacuum to that in the material.
xe2x80x9cOptical associationxe2x80x9d means the reflective coating is positioned relative to the optical elements such that a significant portion of light transmitted through the optical elements can strike the reflective coating and be reflected back into the optical elements.
xe2x80x9cOptical elementsxe2x80x9d are light transmissive elements capable of altering the direction of light that enters the elements so that at least a portion of the light can ultimately be returned towards the light source. The xe2x80x9csizexe2x80x9d of an optical element refers to its characteristic width, depth, height, or length.
xe2x80x9cPolymer layerxe2x80x9d refers to a layer of material that includes organic molecules that have multiple carbon-containing monomer units that are linked in regular or irregular arrangements.
xe2x80x9cReflective coatingxe2x80x9d refers to a coating that is capable of reflecting incident light and that is made up of one or more layers of material.
xe2x80x9cRetroreflectivexe2x80x9d means having the characteristic that obliquely incident incoming light is reflected in a direction antiparallel to the incident direction, or nearly so, such that an observer or detector at or near the light source can detect the reflected light.